1. Field of the Invention
The present invention relates to a light emitting device including an element (hereinafter referred to as an EL element) including a thin film (hereinafter referred to as an organic EL film) which is made of an organic compound to produce EL (Electro Luminescence) and is interposed between electrodes. Particularly, the invention relates to a light emitting device using an organic compound which can convert an energy (hereinafter referred to as a triplet excitation energy) generated in a returning from triplet excitation state to a ground state, into light emission.
2. Description of the Related Art
An EL element using an organic EL film is an element which emits light by applying an electric field, and has attracted attention as an element used for a next generation flat panel display, with characteristics of light weight, low DC voltage driving, high speed response, and the like. Besides, since the element spontaneously emits light and has a wide angle of view, it has been considered to be effective for a display screen of a portable instrument.
In the EL element, an electron injected from a cathode and a hole injected from an anode are recombined at the luminescence center of the organic EL film to form an exciton, and when the exciton returns to the ground state, an energy is radiated to emit light. The excitation state includes a singlet excitation state (S*) and a triplet excitation state (T*), and it has been thought that the statistical generation probability of them is S*:T*=1:3.
However, since light emission (phosphorescence) from the triplet excitation state (T*) is not observed at room temperature in a general organic compound, it has been considered that a theoretical limiting value of an inner quantum efficiency is 25%. Besides, all the generated light can not be radiated to the outside of the element, but part of the light can not be extracted due to a refractive index which is intrinsic to the element. The ratio of the light extracted to the outside of the element to the generated light is called a light extraction efficiency, and it is said that the extraction efficiency is about 20%.
Accordingly, even if all of injected carriers generate excitons, the ratio (external quantum efficiency) of light which can be finally extracted becomes 25%xc3x9720%=5% considering the light extraction efficiency. That is, according to the calculation, even if all the carriers are recombined, only 5% can be extracted as light.
However, recently, an organic material in which the triplet excitation energy can be converted into light emission (phosphorescent emission) is proposed, and its high luminous efficiency attracts attention. As examples in which the triplet exciton is used and the external quantum efficiency is improved, there are following reports.
(1) T. Tsutsui, C Adachi, S. Saito, Photochemical Processes in Organized Molecular Systems, ed. K. Honda, (Elsevier Sci. Pub., Tokyo, 1991) p. 437.
(2) M. A. Baldo, D. F. O""Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson, S. R. Forrest, Nature 395 (1988) p. 151.
(3) M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest, Appl. Phys. Lett., 75 (1999) p.4.
(4) T. Tsutsui, M. -J. Yang, M. Yahiro, K. Nakamura, T. Watanabe, T. Tsuji, Y. Fukuda, T. Wakimoto, S. Mayaguchi, Jpn. Appl. Phys., 38 (12B) (1999) L1502.
Organic materials set forth in the above reports are examples in which the external quantum efficiency is improved by converting the triplet excitation energy into light emission. Among them, there is one which fully exceeds 5% as the theoretical limiting value of the external quantum efficiency described before. For any of the above organic material, a metal complex with platinum, which is a third transition series element, as a center metal (hereinafter referred to as a platinum complex) or a metal complex with iridium as a center material (hereinafter referred to as an iridium complex).
Besides, when a layer made of the iridium complex and a layer made of DCM as a well-known fluorescent pigment are alternately laminated, the triplet excitation energy generated in the iridium complex is transferred to the DCM, and the luminescence center of the DCM is excited so that it is indirectly converted into light emission. Although the light emission of the DCM is light emission (fluorescent emission) from the singlet excitation state, there is a merit that the efficiently generated triplet excitation energy of the iridium complex can be converted to the singlet excitation energy of the DCM.
As described above, the EL element using the organic compound in which the triplet excitation energy can be converted into light emission has a higher external quantum efficiency than that of a conventional one. If the external quantum efficiency is high, light emission brightness is also improved, so that it has been thought that the EL element using the organic compound in which the triplet excitation energy can be converted into light emission has great weight as means for achieving light emission with high brightness in future development.
However, since platinum or iridium is an expensive metal, the metal complexes using those are also expensive, and harmful effects on cost reduction in future is expected. Besides, considering the influence of the metal complex containing a heavy metal upon a human body, a material which is as safe as possible in addition to simplicity of the disposal is desirable as the center metal.
Besides, the luminescent color of the iridium complex is green and the luminescent color of the platinum complex is orange, that is, the both have wavelengths positioned at intermediate places of a visible light region, and red or blue with high color purity has not been obtained. Accordingly, considering the use for a full color flat panel display in future, it is expected that an organic compound becomes necessary; which has a high external quantum efficiency like the iridium complex and the platinum complex and can obtain red light emission and blue light emission with high color purity.
From the above, it can be seen that development of a new organic compound from which phosphorescent can be obtained emission is indispensable in addition to the existing iridium complex and platinum complex.
An object of the present invention is to provide an organic compound in which a triplet excitation energy can be converted into light emission at a lower cost than the prior art.
Another object of the present invention is to provide an organic compound in which a triplet excitation energy can be converted into light emission at a higher efficiency than the prior art.
Still another object of the present invention is to provide an EL element with a high luminous efficiency by using the organic compound.
Yet another object of the present invention is to provide a light emitting device which has brightness and low power consumption by using an EL element with a high luminous efficiency, which is obtained by carrying out the present invention, and an electric instrument using the same and having low power consumption.
The present inventors have paid attention to a heavy atom effect known in photoluminescence (PL). The heavy atom effect indicates a phenomenon in which a spin-orbit interaction is intensified by a heavy atom (atom comprising a lot of atomic nucleus load) introduced in a molecule of an organic compound or a solvent and phosphorescent emission is accelerated. Incidentally, the atomic nucleus load is equivalent to an atomic number, that is, the number of positive electric charges of an atomic nucleus.
Then, the present inventors have considered that it is necessary that the spin-orbit interaction is intense in order to convert the triplet excitation energy into light emission, and an EL element in which the triplet excitation energy can be converted into light emission can be obtained by intentionally using an organic compound containing a molecular structure to intensify the spin-orbit interaction. That is, the present inventors have considered that even if a heavy atom is not used as a center metal, the triplet excitation energy can be converted into light emission provided that an organic compound containing a molecular structure which can produce the heavy atom effect is used.
In order to intensify the spin-orbit interaction without using the heavy atom, it is possible to cite a method of increasing the total atomic nucleus load by increasing the number of center metals to form a molecular structure substantially containing a heavy atom, a method of forming a molecular structure showing ferromagnetism or antiferromagnetism, and a method of forming a molecular structure using a ferromagnetic metal as a center metal. Particularly, it can be said that to form the molecular structure showing ferromagnetism or antiferromagnetism is preferable since the intensity of the spin-orbit interaction can be adjusted by molecular design.
Then, the present invention is characterized in that, as an organic compound containing a molecular structure to intensify the spin-orbit interaction, a polynuclear complex (metal complex including two or more center metals) which has transition elements as center metals is used for an EL element.
That is, the polynuclear complex using a more inexpensive metal (specifically, a first transition series element) than platinum or iridium is used for a light emitting layer (layer made of an organic compound in which recombination is occurred) of an EL element or a dopant pigment (pigment which is added to a light emitting layer and functions as a luminous center), so that the EL element with a high inner quantum efficiency can be obtained at a lower manufacturing cost than the prior art.
Besides, by using the polynuclear complex, the heavy atom is substantially increased as compared with a conventional mononuclear complex using a heavy atom (platinum or iridium), and as a result, the spin-orbit interaction is intensified, and the inner quantum efficiency of the EL element can also be increased.
By using the polynuclear complex, the same or different metals can be made to exist in one metal complex. That is, a molecular structure which a mononuclear complex can not obtain can also be obtained by a combination of center metals, and the selection width of center metals or ligands of the metal complex which emits phosphorescence is widened.
For example, by introducing nickel as a ferromagnetic metal into a metal complex, such as a metal complex with platinum as a center metal, in which phosphorescent emission is originally obtained, there is a possibility that it becomes easy to obtain light emission from the triplet excitation state. Besides, there is a possibility that a transition probability from the triplet excitation state to the singlet excitation state is increased by the introduction of nickel, and there is a possibility that the luminous efficiency of phosphorescent emission can be made higher than the prior art.
Besides, in the case of the polynuclear complex, an excitation energy state is changed by a combination of center metals. In the triplet excitation by the metal complex, there is a case where the center metal receives the triplet excitation energy generated at the ligand and comes to have the triplet excitation state and phosphorescent light is emitted. Accordingly, the luminous color of the phosphorescent light can be changed by changing the excitation energy state through the combination of the center metals.
Further, in the polynuclear complex, it is also possible to design a metal complex in which center metals are combined through a covalent bond. The present inventors consider that it is possible to cause the heavy atom effect when metals in the metal complex exist in a cluster state.
Besides, the polynuclear complex often has a structure in which ligands form a chelate to surround the center metal, and there is a merit that a three-dimensional molecular structure (stable molecular structure) can be easily obtained. Since such three-dimensional structure (also called a steric hindrance) has an effect of suppressing the interaction between molecules, it is possible to eventually made a complex in which it is hard to cause concentration quenching (phenomenon in which light emission disappears when the concentration becomes high).
As described above, the luminous efficiency of the EL element can be made higher than the prior art by making the molecular design while the polynuclear complex is used for the light emitting layer or the dopant pigment. Besides, the luminous color can also be controlled by the molecular design of the polynuclear complex. Besides, as a result, it is possible to obtain a light emitting device which is inexpensive and has low power consumption.
Incidentally, in the present specification, the light emitting device indicates an image display device or a plane emission device using an EL element as a light emitting element. Besides, the light emitting device includes a module in which a TAB (Tape Automated Bonding) tape or a TCP (Tape Carrier Package) is attached to a substrate over which an EL element is formed, a module in which a printed wiring board is provided at an end of a TAB tape or a TCP, and a module in which an IC (Integrated Circuit) is directly mounted on a substrate over which an EL element is formed by a COG (Chip On Glass) system.